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      The term network programming refers to writing programs that execute across multiple devices (computers), in which the devices are all connected to each other using a network.

The java.net package of the J2SE APIs contains a collection of classes and interfaces that provide the low-level communication details, allowing you to write programs that focus on solving the problem at hand.

The java.net package provides support for the two common network protocols:

• TCP: TCP stands for Transmission Control Protocol, which allows for reliable communication between two applications. TCP is typically used over the Internet Protocol, which is referred to as TCP/IP.
• UDP: UDP stands for User Datagram Protocol, a connection-less protocol that allows for packets of data to be transmitted between applications.

This site gives good understanding on the following two subjects:

• Socket Programming: This is most widely used concept in Networking and it has been explained in very detail.
• URL Processing: This would be covered separately. Click here to learn about URL Processing in Java language.

Url Processing

URL stands for Uniform Resource Locator and represents a resource on the World Wide Web, such as a Web page or FTP directory.

This section shows you how to write Java programs that communicate with a URL. A URL can be broken down into parts, as follows:

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protocol://host:port/path?query#ref

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Examples of protocols include HTTP, HTTPS, FTP, and File. The path is also referred to as the filename, and the host is also called the authority.

The following is a URL to a Web page whose protocol is HTTP:

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http://www.amrood.com/index.htm?language=en#j2se

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Notice that this URL does not specify a port, in which case the default port for the protocol is used. With HTTP, the default port is 80.

URL Class Methods:

The java.net.URL class represents a URL and has complete set of methods to manipulate URL in Java.

The URL class has several constructors for creating URLs, including the following:

SN Methods Description 1 public URL(String protocol, String host, int port, String file) throws MalformedURLException. Creates a URL by putting together the given parts. 2 public URL(String protocol, String host, String file) throws MalformedURLException Identical to the previous constructor, except that the default port for the given protocol is used. 3 public URL(String url) throws MalformedURLException Creates a URL from the given String 4 public URL(URL context, String url) throws MalformedURLException Creates a URL by parsing the together the URL and String arguments.

The URL class contains many methods for accessing the various parts of the URL being represented.

Some of the methods in the URL class include the following:

SN Methods Description 1 public String getPath() Returns the path of the URL. 2 public String getQuery() Returns the query part of the URL. 3 public String getAuthority() Returns the authority of the URL. 4 public int getPort() Returns the port of the URL. 5 public int getDefaultPort() Returns the default port for the protocol of the URL. 6 public URLConnection openConnection() throws IOException Opens a connection to the URL, allowing a client to communicate with the resource.

URLConnections Class Methods:

The openConnection() method returns a java.net.URLConnection, an abstract class whose subclasses represent the various types of URL connections.

For example:

• If you connect to a URL whose protocol is HTTP, the openConnection() method returns an HttpURLConnection object.


• If you connect to a URL that represents a JAR file, the openConnection() method returns a JarURLConnection object.


• etc...

The URLConnection class has many methods for setting or determining information about the connection, including the following:

SN Methods Description 1 Object getContent() Retrieves the contents of this URL connection. 2 Object getContent(Class[] classes) Retrieves the contents of this URL connection. 3 String getContentEncoding() Returns the value of the content-encoding header field. 4 int getContentLength() Returns the value of the content-length header field. 5 String getContentType() Returns the value of the content-type header field. 6 int getLastModified() Returns the value of the last-modified header field. 7 long getExpiration() Returns the value of the expires header field. 8 long getIfModifiedSince() Returns the value of this object's ifModifiedSince field. 9 public void setDoInput(boolean input) Passes in true to denote that the connection will be used for input. The default value is true because clients typically read from a URLConnection. 10 public void setDoOutput(boolean output) Passes in true to denote that the connection will be used for output. The default value is false because many types of URLs do not support being written to. 11 public InputStream getInputStream() throws IOException Returns the input stream of the URL connection for reading from the resource. 12 public OutputStream getOutputStream() throws IOException Returns the output stream of the URL connection for writing to the resource 13 public URL getURL() Returns the URL that this URLConnection object is connected to